1.3 Chemical calculations Flashcards
What does the term relative atomic mass mean?
The average mass of one atom of an element compared to 1/12th the mass of an atom of carbon-12.
What does the term relative formula mass mean?
The weighted average of the masses of the formula units compared to 1/12th the mass of an atom of carbon-12.
What does the term relative isotopic mass mean?
The mass of one atom of an isotope compared to 1/12th the mass of an atom of carbon-12.
What is mass spectrometry?
Mass spectrometry is a type of analytical technique which measures the mass to charge ratio of ions. It also measures how abundant each ion is so it can be used to calculate the relative atomic mass.
Explain the five stages of TOF mass spectrometry
- lonisation - vapourised sample is injected into the mass spectrometer where it is ionised.
- Acceleration - the positively charged ions are accelerated towards a negatively charged detection plate.
- lon drift - the ions are deflected into a curved path with a magnetic field.
- Detection - the positive ions are detected by hitting a negatively charged plate, where they gain an electron, producing a current. The greater the current, the greater the abundance.
- Analysis - the relative abundance of each isotope are compared. Relevant calculations are made.
How can you use a mass spectrum to deduce the relative molecular mass of a sample of a compound?
The peak with the highest m/z value (the molecular ion peak, M⁺) is caused by the whole molecule, therefore m/z value = molecular mass.
Define empirical formula
The simplest whole number ratio of atoms of each element in a compound.
Define molecular formula
The actual number of atoms of each element present in a compound.
What is the empirical formula for Fe₂O₄?
FeO₂
What does the term relative molecular mass mean?
The weighted average of the masses of the molecules compared to 1/12th the mass of an atom of carbon-12.
A molecule has the empirical formula C₄H₃O₂ and a relative molecular mass of 166. What is the molecular formula?
Empirical mass = 4(12) + 3(1) + 2(16) = 83
Relative molecular mass = 166
Compare molecular mass with empirical mass = 166 / 83 = 2
So the molecular formula is double the empirical formula: C₈H₆O₂
Define the Avogadro constant
The number of particles in one mole of a substance. This is 6.02 x 10²³ particles.
What is the equation linking Avogadro’s constant to moles?
Number of particles =
Moles x Avogadro’s constant
There are 4.816 x 10²⁴ atoms of iron in a sample.
How many moles of iron are in the sample?
Avogadro’s constant = 6.02 × 10²³ mol⁻¹
Moles = number of atoms ÷ Avogadro’s constant
= (4.816 × 10²⁴) ÷ (6.02 × 10²³)
= 8 mol of Fe
Define molar mass
The mass per mole of a substance, measured in g mol⁻¹.
What is the equation that links moles to mass?
Moles =
Mass (g) x Relative atomic mass
Define concentration
The amount of moles per unit volume.
How can concentration be calculated in g/dm³?
Concentration (g/dm³) =
Mass (g) / Volume (dm³)
How can concentration be calculated in mol/dm³?
Concentration (mol/dm³) =
Moles / Volume (dm³)
5.00 g of NaCl is dissolved in 25 cm³ of water.
Calculate the concentration of the solution in mol/dm³.
Moles of NaCl = 5 / 58.5 = 0.0855
Volume in dm³ = 25 / 1000 = 0.025
Concentration in mol/dm ³ = 0.0855 / 0.025 = 3.42 mol/dm³
What is the molar volume of any gas at room temperature and pressure?
24 dm³
What is RTP?
Room temperature and pressure:
- 20°C
- 1 atmosphere
What equation links molar volume at RTP to moles?
Volume of gas at RTP (dm³) = moles x 24
How many moles of oxygen are in 72 dm³ at RTP?
Moles = volume / 24
= 72 / 24
= 3 moles
How is the molar volume at a given pressure affected if temperature is increased?
An increase in temperature reduces the molar volume at a given temperature.
How is the molar volume at a given temperature affected if pressure is increased?
An increase in pressure will mean the maximum molar volume reduces. Up to this maximum, increasing the pressure will increase the molar volume.
What is the ideal gas law?
pV = nRT
p - pressure (Pa)
V - volume (m³)
n - number of moles
R - gas constant (8.31 J K⁻¹mol⁻¹)
T - temperature (K)
At a temperature of 55.0 °C and a pressure of 275 kPa, a gas occupies a volume of 1.10 dm³. How many moles of the gas are present? The gas constant is 8.31 J K⁻¹ mol⁻¹.
pV = nRT
- 275 kPa is 275000 Pa
- 55.0 °C is 328 K
- 1.10 dm³ is 0.001 m³
Therefore, n=(pV)(RT)
n=(275000 x 0.001)/(8.31 x 328)
n= 0.10 moles
How can percentage yield be calculated?
Percentage yield =
(Actual yield ÷ Theoretical yield) × 100
What is the percentage yield of NH₃, if 40.5 g of NH₃ is produced from 20.0 mol H₂ and excess N₂?
Equation: N₂ + 3H₂ → 2NH₃
Moles of ammonia = 20/1.5 = 13.3 moles
Mass of ammonia = 13.3 x (14+1+1+1) = 227 g
Percentage yield = (40.5/227) × 100 = 17.9%
Why might the actual yield of product be less than expected?
- Incomplete reaction
- Unwanted side reactions
- Practical losses, for example some solid may get lost when being transferred between beakers
What is the atom economy of a reaction?
Atom economy is a measure of the efficiency of the reaction. It looks at the amount of reactants that get turned into useful products.
How can atom economy be calculated?
Atom economy=
(Mr of desired product / total Mr of reactants) × 100
Fill in the gap: ‘The _______ the atom economy, the more sustainable and efficient the process’
Higher
How can percentage error be calculated?
% error = absolute uncertainty/ calculated value x 100
A titre volume is recorded as 11.30 cm³. The accuracy of the burette is ±0.05 cm³. Calculate the maximum percentage error.
% error = absolute uncertainty/ calculated value x 100
(0.05 ÷ 11.30) × 100 = 0.442%
What is meant by stoichiometry?
Stoichiometry is the use of balanced chemical equations to calculate the amount of products and reactants used in the reaction. This mainly uses ratios between the species of the reaction to calculate quantities, such as reacting masses.
